This invention relates to an ignition system for internal combustion engines, or more in particular to an ignition system using a semiconductor element, especially a power transistor as a switching element for the primary winding circuit of the ignition coil.
In conventional ignition systems utilizing the opening and closing of a mechanical contact or breaker point, a capacitor of about 0.15 .mu.F to 0.25 .mu.F is connected in parallel to the breaker point. This capacitor affects the opening speed of the breaker point but not the closing speed thereof, i.e., the speed at which the source voltage is applied to the ignition coil. Therefore, simultaneously with the closing of the breaker point, the source voltage is substantially applied across the primary winding of the ignition coil.
In an electronic ignition system including a transistor ignition system having a semiconductor element such as a transistor or thyristor as the switching element for the primary winding circuit of the ignition coil, the operating speed of the switching transistor is such that the time from the turning off of the transistor to the application of predetermined voltage to the primary circuit of the ignition coil is about 1 to 5 microseconds, and the transistor is turned on at substantially the stable speed of 5 to 40 microseconds. Therefore, at the same time that the switching transistor is turned on, the source voltage is applied across the primary winding of the ignition coil. In another type of electronic ignition system having a thyristor as the switching element, a surge absorber including a series circuit of a resistor and a capacitor is connected in parallel to the thyristor. This series circuit reduces the surge but fails to control the switching speed of the thyristor.
As mentioned above, regardless of whether the mechanical contact or the electronic device such as transistor or thyristor is used as the switching element, the speed at which the switching element is turned on is not controlled. As soon as the switching element is turned on, therefore, the source voltage less the saturation voltage of the switching element is applied across the primary winding of the ignition coil, so that the primary current determined by the primary winding and an external resistance making up the primary circuit starts to flow in the ignition coil. Although in the prior art attention is paid only to the fact that a high voltage is generated in the secondary winding of the ignition coil by the turning off of the switching element and the resulting cutoff of the primary circuit, a transient phenomenon occurs of course also at the time of closing of the primary winding circuit, with the result that the secondary high voltage is generated in the secondary side of the ignition coil. The secondary high voltage generated in the secondary winding in response to the closing of the primary winding circuit of the ignition coil is reverse in polarity to the secondary high voltage generated in response to the opening of the primary winding circuit, but they are substantially equal to each other in the frequency of output voltage. Such an output voltage waveform is shown in FIG. 1. In this figure, a and c show output voltage waveforms generated in the secondary winding of the ignition coil at the ignition timings. Specifically, a shows an open waveform for the secondary side without generation of any spark, c a waveform associated with break with generation of a spark, and b a waveform associated with conduction of the primary side of the ignition coil. The secondary high voltage, i.e., V2 in FIG. 1 at point b takes a value about one tenth of the output voltage level at the ignition timings, but it sometimes takes a voltage from 2 to 4 kv depending on the ignition coil when the source voltage is 14 volts. The source voltage characteristics of the high voltage generated in the secondary winding when the primary winding circuit of the ignition coil is closed is shown in FIG. 2. This secondary high voltage increases substantially in proportion to the source voltage, and has different output levels depending on the variety of the ignition coil as shown by a, b and c.
As described above, a secondary high voltage is generated in the secondary winding even when the primary winding circuit of the ignition coil is closed. This secondary high voltage sometimes undesirably causes the ignition plug to be fired. This is because the secondary high voltage is connected to the ignition plug generally through a distributor or, directly in the case lacking the distributor. The breakage of the secondary high voltage and the resulting firing at a point different from normal ignition timing is liable to cause irrevocable damage to the engine. In most of the distributors of rotary type, the angular point when the ignition coil is turned on is fixed. Namely, in most case the ignition coil is turned on almost when the rotor in a distributor comes to the center between the distribution terminals and therefore a considerable air gap exists between rotor and distributor terminal, thus substantially but not entirely eliminating the possibility of actuation of the ignition plug. In the ignition system in which the closing angle is subjected to variable control, the position b in FIG. 1 may be located at a given point between points a and c, so that the air gap is not sufficiently large and the breakage of ignition plug causes trouble in the engine, thus making it necessary to take such measures as enlarging the diameter of the high voltage distribution section of distributor. In the ignition system in which the secondary high voltage is directly applied to the ignition plug, on the other hand, the turning on of the switching element and the resulting high secondary voltage generated across the secondary winding of course causes trouble in engine operation. Further, the increased arc energy, the increased arc current and the increased secondary voltage due to the grading up of the ignition system to high performance leads to large amplification of the secondary high voltage across the secondary winding from the voltage applied across the primary winding, further increasing the chance of engine trouble.
The present inventors have conducted a test using such a high-performance ignition system, applying a voltage across the primary winding of ignition coil by turning on the switching element, and observing the arc generated by the ignition plug being fired. The results of this test show that the arc generated when the switching element is turned on has substantially the same duration, arc current and arc energy as the arc generated at normal ignition timing and that the generation of this abnormal arc adversely affects the engine. The secondary high voltage waveform and arc current waveform generated when an arc is generated by turning on the switching element and when the switching element is turned off with the ignition coil primary current cut off are shown in FIG. 3 respectively.
The adverse effects on the engine are shown in the diagram of FIG. 4 illustrating pressure in cylinder. In this drawing, in case of normal ignition, the pressure peak appears about 10 degrees after the top dead center as shown by solid line, while the pressure peak point in case of arc generation in response to the turning on of the switching element is reached almost at the top dead center as shown by dashed line, and the pressure level at that time is twice or thrice the normal pressure, resulting in a higher rate of pressure increase. At the time of generation of the pressure shown by the dashed line, the engine torque is reduced sharply on the one hand and the engine may be seriously damaged on the other hand. Other disadvantages that may occur in such a case include knocking, after-burn and other undesirable phenomena.